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Photoelectrochemical hydrogen production from water using p-type and n-type oxide semiconductor electrodes
Electrochimica Acta 82 (2012) 397–401
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Photoelectrochemical hydrogen production from water using p-type and n-type oxide semiconductor electrodes Shintaro Ida a,b,∗ , Keisuke Yamada b , Maki Matsuka a , Hidehisa Hagiwara a , Tatsumi Ishihara a a b
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
a r t i c l e
i n f o
Article history: Received 30 November 2011 Received in revised form 28 March 2012 Accepted 28 March 2012 Available online 12 April 2012 Keywords: Photoelectrochemical Water splitting CaFe2 O4 TiO2 Artificial photosynthesis
a b s t r a c t Photoelectrochemical hydrogen and oxygen production from water were demonstrated without external voltage using calcium iron oxide (p-type) and TiO2 (n-type) semiconductor electrodes. The calcium iron oxide electrode with the ratio Fe/Ca of 1.9, which consisted of two crystal phases (main phase: CaFe2 O4 , impurity phase: Ca2 Fe2 O5 ), showed the highest photocathodic current in 0.1 M NaOH aq. at potentials below +0.30 V vs. Ag/AgCl under a 500 W Xe lamp illumination, and TiO2 showed photo-anodic current in 0.1 M NaOH aq. at potentials over −0.78 V vs. Ag/AgCl. In the system where the two electrodes were connected under illumination, the open-circuit voltage was 1.09 V and the short-circuit current density was 550 A cm−2 . Hydrogen and oxygen were successfully generated from this present system without applying an external voltage. The ratio of hydrogen/oxygen evolved after 12-h reaction with both the electrodes short-circuited was around 3.7. This system is an ultimately artificial photosynthesis system where hydrogen and oxygen are generated separately under illumination. It is believed that this present system can form the basis of the future artificial photosynthesis system where hydrogen and oxygen are produced directly from water and sunlight. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction The generation of hydrogen from water using solar energy has attracted considerable interests, due to its clean and environmentally friendly nature of energy generation. Photoelectrochemical (PEC) hydrogen production using semiconductor electrodes is one of the promising methods of hydrogen production from water using solar energy [1,2]. The advantage of the PEC process is that photooxidation and photo-reduction sites are apart, which allows the separate collection of hydrogen and oxygen. The first report on the PEC hydrogen production was a system with a n-type semiconductor, TiO2 –Pt system [1]. Some recent studies have focused on high-efficiency n-type semiconductor photoanodes, such as TaON [3] and Fe2 O3 [4], and high-efficiency p-type semiconductor photocathodes, such as Cu(In,Ga)Se2 [5] and Cu2 ZnSnS4 [6]. However, semiconductor metal electrodes systems such as a TiO2 –Pt system generally require an external voltage for the hydrogen generation process to take place. The development of a PEC hydrogen
∗ Corresponding author at: Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. Tel.: +81 92 802 2869; fax: +81 92 802 2870/71. E-mail address: [email protected] (S. Ida). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.03.174
generation system which does not require any external voltage is crucial for the efficient use of solar energy. Such a system is ultimately an artificial photosynthesis system that can generate hydrogen and oxygen separately, and is required to satisfy the following conditions: (1) the flat-band potential of p-type semiconductor is more positive than the n-type; (2) the position of conduction band of the p-type semiconductor is more negative than the reduction potential of H2 O (0 V vs. SHE); and (3) the position of valence band of the n-type semiconductor is more positive than the oxidation level of H2 O (1.23 V vs. SHE). So far, various electrode systems such as Mg-doped Fe2 O3 (p-type)–Sidoped (n-type) [7] and SiC (p-type)–SiC (n-type) [8] systems have been reported as systems without any external voltage. However, there are only a few electrode systems in which hydrogen and oxygen evolve at the theoretical ratio (H2 /O2 = 2) with a high Faraday efficiency. Our previous study [9] examined a PEC hydrogen production system using CaFe2 O4 . The CaFe2 O4 is a p-type semiconductor with a band gap of 1.9 eV (the conduction and valence band edges of −0.6 and +1.3 V vs. SHE, respectively [10]), and the position of the conduction band is suitable for reduction of water. In addition, it can be prepared from low-cost materials such as iron and calcium. In the study, hydrogen and oxygen were generated from a photocell consisting CaFe2 O4 and TiO2 electrodes (CaFe2 O4 –TiO2 system) under
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illumination without applying any external voltage [9]. However, the ratio of H2 /O2 evolved was only 10–20, and the perfect water splitting reaction (H2 /O2 = 2) was not achieved. In this paper, we report the photoelectrochemical production of hydrogen and oxygen from water without external applied voltage using CaFe2 O4 –Ca2 Fe2 O5 and TiO2 semiconductor electrodes. The ratio of hydrogen/oxygen evolved in the system was close to the theoretical value for the complete water splitting (H2 /O2 = 2).
2. Experimental The calcium iron oxide electrodes were prepared by, first, calcium acetate monohydrate (99%, Wako Pure Chemical Industries, Ltd.) and iron (III) nitrate enneahydrate (99.9%, Wako Pure Chemical Industries, Ltd.) in deionized water, followed by the addition of an aqueous solution of polyethylene glycol of 5 wt.% to the mixture [9]. The solution was stirred and heated at 120 ◦ C until fully dried. The powder was then calcined at 450 ◦ C for 2 h, and at 1050 ◦ C for 10 h. The powder obtained (50 mg) was suspended in 200 mL of ethanol, and the suspension was applied onto a Pt substrate (1 cm × 2 cm), and dried at 80 ◦ C. The substrate was calcined at 1200 ◦ C for 2 h. TiO2 electrode (1 cm × 2 cm or 0.5 cm × 0.5 cm) was prepared by annealing a metal titanium (thickness: 0.1 mm, purity: 99.5%, Nilaco Corp.) at 600 ◦ C for 1 h. The electrochemical experiments were carried out in a conventional three-electrode electrochemical cell with a Pt counter electrode and a Ag/AgCl (saturated KCl) reference electrode. A 500 W Xe lamp (USHIO SX-UI500XQ) with 17 mW cm−2 UV-light intensity was used as the light source. Bandpass filters and long pass filters (ASAHI SPECTRA: MZ0320, MZ0350, MZ0380, MZ0400, MZ0450, and MZ0500) were used for the evaluation of apparent quantum efficiency and photocatalytic activity under visible light illumination. The PEC hydrogen production experiment was carried out in 0.1 M NaOH aqueous solution under illumination with the 500-W Xe lamp. The back side of the electrode was covered with masking tape to prevent the contact with NaOH electrolyte solution. The experimental apparatus for the hydrogen generation consisted of two chambers where the anode and cathode electrodes were immersed in separate quartz cells. The two chambers were connected through a Nafion 117 film. The experiment was carried out in an Ar gas atmosphere (approximately 150 Torr).
Fig. 1. XRD pattern of calcium iron oxides with various Fe/Ca ratios: (a) 2.1, (b) 2.0, (c) 1.9, and (d) 1.8.
Fig. 2. XRD patterns of calcium iron oxide films after calcination at 1200 ◦ C: (a) Fe/Ca = 2.0, (b) Fe/Ca = 1.9, and (c) Ca2 Fe2 O5 powder.
The crystal structure of the powder was analyzed using X-ray diffraction (XRD) analysis (Cu-K␣ radiation, Rigaku RINT2500VHF). The surface morphology and elemental distribution were analyzed using a scanning electron microscope (SEM) (VE7800, KEYENCE) and energy dispersive X-ray spectroscopy (EDX).
Fig. 3. SEM images of CFO electrodes with (a) Fe/Ca = 2.0 and (b) 1.9.
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Fig. 4. EDX spectra and mapping results of CFO electrode with Fe/Ca = 1.9: (a) SEM image, (b) EDX spectra, (c) EDX mapping of Ca-K␣, and (d) EDX mapping of Fe-K␣.
The H2 and O2 evolution rates were determined using a gas chromatograph (Shimadzu GC-8A, TCD, Ar carrier, MS-5A). 3. Results and discussion Fig. 1 shows the XRD patterns of calcium iron oxide (CFO) powder with various Fe/Ca ratios obtained after calcination at 1050 ◦ C. The diffraction pattern of the powder with Fe/Ca ratio of 2.0 corresponded to that of CaFe2 O4 , which has an orthorhombic phase ˚ [11]. The devi(space group: Pnam; a = 9.23, b = 10.71, c = 3.02 A) ations of the Fe/Ca ratio from 2.0 resulted in the formation of impurity phases, Fe2 O3 or Ca2 Fe2 O5 . The thickness of the CFO films deposited on the Pt electrode were approximately 50–100 m. Fig. 2 shows the XRD patterns of CFO films with the Fe/Ca ratio of 2.0 and 1.9, prepared at 1200 ◦ C, along with the precursor powder. All the films showed several intensive diffraction peaks and weak peaks. In the case of the CFO (Fe/Ca = 2.0) film, the intense peaks at around 33 degree were assigned to (3 2 0) and (0 4 0) peaks of CaFe2 O4 , and the weak peaks were assigned to other (h k 0) diffraction peaks of CaFe2 O4 . The CFO film was removed from the Pt electrode and ground. The XRD pattern of the ground CFO powder (Fe/Ca = 2.0) showed the same diffraction peaks as the CaFe2 O4 powder (Fig. 1) without any impurity phase. These results indicate that the calcination at 1200 ◦ C produced the (h k 0)-oriented CaFe2 O4 film. The CFO electrodes prepared with the Fe/Ca ratio of 1.9 contained Ca2 Fe2 O5 impurity phase. Fig. 3a and b shows the SEM images of the CFO electrodes, Fe/Ca = 2.0 and 1.9, respectively. The microstructure of CFO (Fe/Ca = 2.0) surface was flat and smooth, while the CFO (Fe/Ca = 1.9) contained small irregular rectangular particles emerged from the bulk, which may be Ca2 Fe2 O5 particles. The EDX spectrum imaging was performed on the surface of the CFO (Fe/Ca = 1.9) electrode to characterize the elemental distribution (Fig. 4). The peak intensity
of Ca-K␣ signal from the rectangular particle site was larger than that from the smooth site, and then the peak intensity of Fe-K␣ signal from the rectangular particles site was smaller than that from the smooth site. This indicates that the rectangular particles are Ca2 Fe2 O5 . Fig. 5a shows the photocathodic currents of the CFO electrodes as a function of the Fe/Ca ratio. The CFO electrode with the
Fig. 5. (a) Photo-cathodic currents of the CFO electrodes under illumination as a function of Fe/Ca ratios, and (b) current-potential curves of CFO electrode (Fe/Ca = 1.9) under full arc and visible light (cut-off < 420 nm) illumination with a 500 W Xe lamp.
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Fig. 6. Incident photon-to-current efficiency spectrum of CFO (Fe/Ca = 1.9) in 0.1 M NaOH at −0.4 V vs. Ag/AgCl.
Fe/Ca ratio of 1.9 showed the highest photocathodic current. It is suspected that Ca2 Fe2 O5 impurity phase in the CFO (Fe/Ca = 1.9) electrode may have contributed to the improved photocurrent. To investigate the effect of Ca2 Fe2 O5 on the photocurrent, a CFO film with the composition, Ca2 Fe2 O5 /CaFe2 O4 = 1/18, which is equivalent to Fe/Ca ratio of 1.9, was prepared using CaFe2 O4 and Ca2 Fe2 O5 powder. As expected, the photocurrent of this CFO electrode was higher than that of the pure CaFe2 O4 film. The mechanism for this improved photocurrent in Ca2 Fe2 O5 /CaFe2 O4 systems is not yet fully understood and currently under investigation. Fig. 5b shows the current-potential curves for the CFO electrode (Fe/Ca = 1.9) under full arc and visible light (>420 nm) using the Xe lamp. The CFO electrode showed cathodic photocurrents under the visible light illumination. Fig. 6 shows an incident photon-to-current efficiency (IPCE) spectrum in 0.1 M NaOH at −0.4 V vs. Ag/AgCl. The IPCE was 2–4% in the UV region, and less than 1% in the visible light region. Although a photocurrent was observed up to 600 nm (bandgap of CFO: 1.9 eV), the photocathodic current at 650 nm was less than 0.01%. This photocurrent is due to the photoreduction of water to hydrogen. The onset potentials under full arc and visible light were both +0.30 V vs. Ag/AgCl. The flat band potential was roughly estimated by plotting the onset potential and the pH. The slope of the line was −60 mV, and the potential at pH = 0 was estimated to be 1.05 V (1.25 V) vs. Ag/AgCl (vs. SHE), which was close to the value for a pure CaFe2 O4 electrode reported previously [10]. In general, the onset potential is close to the flat-band potential and the valence band is close to the flat-band potential in p-type semiconductors. Fig. 7 shows the Mott–Schottky plot of the CFO (Fe/Ca = 1.9) electrode in 0.5 M H2 SO4 solution. The value of the flat band potential estimated from the plot was 1.19 V, which was in good accordance with the one determined from the onset potential (1.25 V). The valence band of CaFe2 O4 pallets was reported as around 1.3 V vs. SHE [10]. The onset potential of TiO2 (rutile) electrode in 0.1 M NaOH solution was −0.78 V. Fig. 8 shows the current-potential curve for a cell where p-type CFO (Fe/Ca = 1.9) and n-type TiO2 electrodes were connected in 0.1 M NaOH aqueous solution in the two chamber system under the Xe lamp illumination (full arc). The OCV was 1.09 V, and the shortcircuit current (V = 0) was 550 A. The difference between the two onset potentials of CaFe2 O4 (0.30 V) and TiO2 (−0.78 V) was 1.08 V, which was close to the OCV of the photocell in Fig. 8. The maximum power density was 0.108 mW cm−2 . Fig. 9a shows the amounts of hydrogen and oxygen generated in the photocell with CFO (Fe/Ca = 1.9) and TiO2 electrodes under short circuited condition, plotted as a function of the irradiation time. The amount of H2 evolved was approximately equivalent to a half of the electrons passing through the outer circuit, while the amount
Fig. 7. Mott–Schottky plot of the CFO (Fe/Ca = 1.9) electrode in 0.5 M H2 SO4 solution.
Fig. 8. Current-potential curves for a photocell with p-type CFO (Fe/Ca = 1.9) and n-type TiO2 electrodes in 0.1 M NaOH aqueous solution and powder density under a Xe lamp full arc illumination (electrode area of CFO: 2 cm2 , electrode area of TiO2 : 2 cm2 ).
of O2 evolved was equivalent to less than a quarter of the electron transport. The Faraday efficiencies for the H2 and O2 generation were 96–100% and 33–51%, which may be due to the oxidation of Ti substrate or oxygen absorption on the TiO2 /Ti electrode. The gas production rates decreased with the increasing irradiation time. Table 1 shows the ratio of hydrogen and oxygen evolved from the short circuited CFO–TiO2 electrodes. The ratio increased from 3.7 to 6.1 with the reaction time. The reason for the deviation of H2 /O2 ratio from 2.0 might be due to the photo-decomposition of CFO electrode. Fig. 9b shows the current-time curve for the CFO (Fe/Ca = 1.9)–TiO2 system. The photocurrent slowly decreased with Table 1 Ratio of hydrogen and oxygen evolved from the short circuited CFO–TiO2 electrodes. Reaction time [h]
H2 /O2 ratio
12 24 36 54
3.7 6.1 6.2 6.1
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Fig. 9. (a) Amounts of H2 and O2 gases generated from CFO (Fe/Ca = 1.9) and TiO2 electrodes system as a function of illumination time, and (b) current-time curve for the CFO-TiO2 system (electrode area of CFO: 2 cm2 , electrode area of TiO2 : 0.25 cm2 ).
detected under visible light illumination (>420 nm). If a n-type semiconductor with visible light response is used instead of the TiO2 electrode, hydrogen will be generated with this present system without any external voltage under visible light illumination. 4. Conclusions
Fig. 10. SEM image of CFO (Fe/Ca = 1.9) electrode after 52 h reaction. Table 2 Incident photon-to-current efficiency for the short-circuited CFO (Fe/Ca = 1.9)–TiO2 system under monochromatic light. Wavelength [nm]
Power density [mW/cm2 ]
IPCE [%]
320 350 370 400 420 450 470 500
3.917 4.207 4.757 8.725 9.894 10.769 11.185 9.557
2.02 1.20 0.66 0.06 0.02 0.01 0.01 0.01
the illumination time. Fig. 10 shows SEM image of the CFO electrode after a 52-h reaction. Many cracks were observed on the CFO surface, indicating that the CFO electrode slightly decomposed during the reaction, which may have led to the reduction of the photocurrent. Table 2 shows the incident photon-to-current efficiency for the short-circuited CFO (Fe/Ca = 1.9)–TiO2 system under monochromatic light. The efficiency at 320 nm was 2.02%, and decreased with increasing light wavelength. Hydrogen gas was not
Hydrogen and oxygen were generated from the p-type CFO (Fe/Ca = 1.9) and n-type TiO2 electrode system without any external voltage under illumination. It was found that the present CFO (Fe/Ca = 1.9) electrode showed superior performance to the pure CaFe2 O4 electrode. The ratio of H2 /O2 evolved from the CFO (Fe/Ca = 1.9)–TiO2 system after a 12-h reaction was 3.7. To the best of our knowledge, the present result is the closest to the complete water splitting using oxide p-type and n-type semiconductor electrodes. This present system using p-type and n-type semiconductor materials is a promising method of hydrogen production using solar energy. It is believed that this present system can form the basis of the future artificial photosynthesis system where hydrogen and oxygen are produced directly from water and sunlight. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Chemical Reviews 110 (2011) 6446. [3] R. Abe, M. Higashi, K. Domen, Journal of the American Chemical Society 132 (2010) 11828. [4] I. Cesar, A. Kay, J.A.G. Martinez, M. Grätzel, Journal of the American Chemical Society 128 (2006) 4582. [5] D. Yokoyama, T. Minegishi, K. Maeda, M. Katayama, J. Kubota, A. Yamada, M. Konagai, K. Domen, Electrochemistry Communications 12 (2010) 851. [6] G. Ma, T. Minegishi, D. Yokoyama, J. Kubota, K. Domen, Chemical Physics Letters 501 (2011) 619. [7] J.E. Turner, M. Hendewerk, G.A. Somorjai, Chemical Physics Letters 105 (1984) 581. [8] T. Inoue, T. Yamase, Chemistry Letters (1985) 869. [9] S. Ida, Y. Yamada, T. Matsunaga, H. Hagiwara, Y. Matsumoto, T. Ishihara, Journal of the American Chemical Society 132 (2010) 17343. [10] Y. Matsumoto, M. Omae, K. Sugiyama, E. Sato, Journal of Physical Chemistry 91 (1987) 577. [11] B.F. Decker, J.S. Kasper, Acta Crystallographica 10 (1957) 332.
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Photoelectrochemical hydrogen production from water using p-type and n-type oxide semiconductor electrodes Shintaro Ida a,b,∗ , Keisuke Yamada b , Maki Matsuka a , Hidehisa Hagiwara a , Tatsumi Ishihara a a b
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
a r t i c l e
i n f o
Article history: Received 30 November 2011 Received in revised form 28 March 2012 Accepted 28 March 2012 Available online 12 April 2012 Keywords: Photoelectrochemical Water splitting CaFe2 O4 TiO2 Artificial photosynthesis
a b s t r a c t Photoelectrochemical hydrogen and oxygen production from water were demonstrated without external voltage using calcium iron oxide (p-type) and TiO2 (n-type) semiconductor electrodes. The calcium iron oxide electrode with the ratio Fe/Ca of 1.9, which consisted of two crystal phases (main phase: CaFe2 O4 , impurity phase: Ca2 Fe2 O5 ), showed the highest photocathodic current in 0.1 M NaOH aq. at potentials below +0.30 V vs. Ag/AgCl under a 500 W Xe lamp illumination, and TiO2 showed photo-anodic current in 0.1 M NaOH aq. at potentials over −0.78 V vs. Ag/AgCl. In the system where the two electrodes were connected under illumination, the open-circuit voltage was 1.09 V and the short-circuit current density was 550 A cm−2 . Hydrogen and oxygen were successfully generated from this present system without applying an external voltage. The ratio of hydrogen/oxygen evolved after 12-h reaction with both the electrodes short-circuited was around 3.7. This system is an ultimately artificial photosynthesis system where hydrogen and oxygen are generated separately under illumination. It is believed that this present system can form the basis of the future artificial photosynthesis system where hydrogen and oxygen are produced directly from water and sunlight. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction The generation of hydrogen from water using solar energy has attracted considerable interests, due to its clean and environmentally friendly nature of energy generation. Photoelectrochemical (PEC) hydrogen production using semiconductor electrodes is one of the promising methods of hydrogen production from water using solar energy [1,2]. The advantage of the PEC process is that photooxidation and photo-reduction sites are apart, which allows the separate collection of hydrogen and oxygen. The first report on the PEC hydrogen production was a system with a n-type semiconductor, TiO2 –Pt system [1]. Some recent studies have focused on high-efficiency n-type semiconductor photoanodes, such as TaON [3] and Fe2 O3 [4], and high-efficiency p-type semiconductor photocathodes, such as Cu(In,Ga)Se2 [5] and Cu2 ZnSnS4 [6]. However, semiconductor metal electrodes systems such as a TiO2 –Pt system generally require an external voltage for the hydrogen generation process to take place. The development of a PEC hydrogen
∗ Corresponding author at: Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. Tel.: +81 92 802 2869; fax: +81 92 802 2870/71. E-mail address: [email protected] (S. Ida). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.03.174
generation system which does not require any external voltage is crucial for the efficient use of solar energy. Such a system is ultimately an artificial photosynthesis system that can generate hydrogen and oxygen separately, and is required to satisfy the following conditions: (1) the flat-band potential of p-type semiconductor is more positive than the n-type; (2) the position of conduction band of the p-type semiconductor is more negative than the reduction potential of H2 O (0 V vs. SHE); and (3) the position of valence band of the n-type semiconductor is more positive than the oxidation level of H2 O (1.23 V vs. SHE). So far, various electrode systems such as Mg-doped Fe2 O3 (p-type)–Sidoped (n-type) [7] and SiC (p-type)–SiC (n-type) [8] systems have been reported as systems without any external voltage. However, there are only a few electrode systems in which hydrogen and oxygen evolve at the theoretical ratio (H2 /O2 = 2) with a high Faraday efficiency. Our previous study [9] examined a PEC hydrogen production system using CaFe2 O4 . The CaFe2 O4 is a p-type semiconductor with a band gap of 1.9 eV (the conduction and valence band edges of −0.6 and +1.3 V vs. SHE, respectively [10]), and the position of the conduction band is suitable for reduction of water. In addition, it can be prepared from low-cost materials such as iron and calcium. In the study, hydrogen and oxygen were generated from a photocell consisting CaFe2 O4 and TiO2 electrodes (CaFe2 O4 –TiO2 system) under
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illumination without applying any external voltage [9]. However, the ratio of H2 /O2 evolved was only 10–20, and the perfect water splitting reaction (H2 /O2 = 2) was not achieved. In this paper, we report the photoelectrochemical production of hydrogen and oxygen from water without external applied voltage using CaFe2 O4 –Ca2 Fe2 O5 and TiO2 semiconductor electrodes. The ratio of hydrogen/oxygen evolved in the system was close to the theoretical value for the complete water splitting (H2 /O2 = 2).
2. Experimental The calcium iron oxide electrodes were prepared by, first, calcium acetate monohydrate (99%, Wako Pure Chemical Industries, Ltd.) and iron (III) nitrate enneahydrate (99.9%, Wako Pure Chemical Industries, Ltd.) in deionized water, followed by the addition of an aqueous solution of polyethylene glycol of 5 wt.% to the mixture [9]. The solution was stirred and heated at 120 ◦ C until fully dried. The powder was then calcined at 450 ◦ C for 2 h, and at 1050 ◦ C for 10 h. The powder obtained (50 mg) was suspended in 200 mL of ethanol, and the suspension was applied onto a Pt substrate (1 cm × 2 cm), and dried at 80 ◦ C. The substrate was calcined at 1200 ◦ C for 2 h. TiO2 electrode (1 cm × 2 cm or 0.5 cm × 0.5 cm) was prepared by annealing a metal titanium (thickness: 0.1 mm, purity: 99.5%, Nilaco Corp.) at 600 ◦ C for 1 h. The electrochemical experiments were carried out in a conventional three-electrode electrochemical cell with a Pt counter electrode and a Ag/AgCl (saturated KCl) reference electrode. A 500 W Xe lamp (USHIO SX-UI500XQ) with 17 mW cm−2 UV-light intensity was used as the light source. Bandpass filters and long pass filters (ASAHI SPECTRA: MZ0320, MZ0350, MZ0380, MZ0400, MZ0450, and MZ0500) were used for the evaluation of apparent quantum efficiency and photocatalytic activity under visible light illumination. The PEC hydrogen production experiment was carried out in 0.1 M NaOH aqueous solution under illumination with the 500-W Xe lamp. The back side of the electrode was covered with masking tape to prevent the contact with NaOH electrolyte solution. The experimental apparatus for the hydrogen generation consisted of two chambers where the anode and cathode electrodes were immersed in separate quartz cells. The two chambers were connected through a Nafion 117 film. The experiment was carried out in an Ar gas atmosphere (approximately 150 Torr).
Fig. 1. XRD pattern of calcium iron oxides with various Fe/Ca ratios: (a) 2.1, (b) 2.0, (c) 1.9, and (d) 1.8.
Fig. 2. XRD patterns of calcium iron oxide films after calcination at 1200 ◦ C: (a) Fe/Ca = 2.0, (b) Fe/Ca = 1.9, and (c) Ca2 Fe2 O5 powder.
The crystal structure of the powder was analyzed using X-ray diffraction (XRD) analysis (Cu-K␣ radiation, Rigaku RINT2500VHF). The surface morphology and elemental distribution were analyzed using a scanning electron microscope (SEM) (VE7800, KEYENCE) and energy dispersive X-ray spectroscopy (EDX).
Fig. 3. SEM images of CFO electrodes with (a) Fe/Ca = 2.0 and (b) 1.9.
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Fig. 4. EDX spectra and mapping results of CFO electrode with Fe/Ca = 1.9: (a) SEM image, (b) EDX spectra, (c) EDX mapping of Ca-K␣, and (d) EDX mapping of Fe-K␣.
The H2 and O2 evolution rates were determined using a gas chromatograph (Shimadzu GC-8A, TCD, Ar carrier, MS-5A). 3. Results and discussion Fig. 1 shows the XRD patterns of calcium iron oxide (CFO) powder with various Fe/Ca ratios obtained after calcination at 1050 ◦ C. The diffraction pattern of the powder with Fe/Ca ratio of 2.0 corresponded to that of CaFe2 O4 , which has an orthorhombic phase ˚ [11]. The devi(space group: Pnam; a = 9.23, b = 10.71, c = 3.02 A) ations of the Fe/Ca ratio from 2.0 resulted in the formation of impurity phases, Fe2 O3 or Ca2 Fe2 O5 . The thickness of the CFO films deposited on the Pt electrode were approximately 50–100 m. Fig. 2 shows the XRD patterns of CFO films with the Fe/Ca ratio of 2.0 and 1.9, prepared at 1200 ◦ C, along with the precursor powder. All the films showed several intensive diffraction peaks and weak peaks. In the case of the CFO (Fe/Ca = 2.0) film, the intense peaks at around 33 degree were assigned to (3 2 0) and (0 4 0) peaks of CaFe2 O4 , and the weak peaks were assigned to other (h k 0) diffraction peaks of CaFe2 O4 . The CFO film was removed from the Pt electrode and ground. The XRD pattern of the ground CFO powder (Fe/Ca = 2.0) showed the same diffraction peaks as the CaFe2 O4 powder (Fig. 1) without any impurity phase. These results indicate that the calcination at 1200 ◦ C produced the (h k 0)-oriented CaFe2 O4 film. The CFO electrodes prepared with the Fe/Ca ratio of 1.9 contained Ca2 Fe2 O5 impurity phase. Fig. 3a and b shows the SEM images of the CFO electrodes, Fe/Ca = 2.0 and 1.9, respectively. The microstructure of CFO (Fe/Ca = 2.0) surface was flat and smooth, while the CFO (Fe/Ca = 1.9) contained small irregular rectangular particles emerged from the bulk, which may be Ca2 Fe2 O5 particles. The EDX spectrum imaging was performed on the surface of the CFO (Fe/Ca = 1.9) electrode to characterize the elemental distribution (Fig. 4). The peak intensity
of Ca-K␣ signal from the rectangular particle site was larger than that from the smooth site, and then the peak intensity of Fe-K␣ signal from the rectangular particles site was smaller than that from the smooth site. This indicates that the rectangular particles are Ca2 Fe2 O5 . Fig. 5a shows the photocathodic currents of the CFO electrodes as a function of the Fe/Ca ratio. The CFO electrode with the
Fig. 5. (a) Photo-cathodic currents of the CFO electrodes under illumination as a function of Fe/Ca ratios, and (b) current-potential curves of CFO electrode (Fe/Ca = 1.9) under full arc and visible light (cut-off < 420 nm) illumination with a 500 W Xe lamp.
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Fig. 6. Incident photon-to-current efficiency spectrum of CFO (Fe/Ca = 1.9) in 0.1 M NaOH at −0.4 V vs. Ag/AgCl.
Fe/Ca ratio of 1.9 showed the highest photocathodic current. It is suspected that Ca2 Fe2 O5 impurity phase in the CFO (Fe/Ca = 1.9) electrode may have contributed to the improved photocurrent. To investigate the effect of Ca2 Fe2 O5 on the photocurrent, a CFO film with the composition, Ca2 Fe2 O5 /CaFe2 O4 = 1/18, which is equivalent to Fe/Ca ratio of 1.9, was prepared using CaFe2 O4 and Ca2 Fe2 O5 powder. As expected, the photocurrent of this CFO electrode was higher than that of the pure CaFe2 O4 film. The mechanism for this improved photocurrent in Ca2 Fe2 O5 /CaFe2 O4 systems is not yet fully understood and currently under investigation. Fig. 5b shows the current-potential curves for the CFO electrode (Fe/Ca = 1.9) under full arc and visible light (>420 nm) using the Xe lamp. The CFO electrode showed cathodic photocurrents under the visible light illumination. Fig. 6 shows an incident photon-to-current efficiency (IPCE) spectrum in 0.1 M NaOH at −0.4 V vs. Ag/AgCl. The IPCE was 2–4% in the UV region, and less than 1% in the visible light region. Although a photocurrent was observed up to 600 nm (bandgap of CFO: 1.9 eV), the photocathodic current at 650 nm was less than 0.01%. This photocurrent is due to the photoreduction of water to hydrogen. The onset potentials under full arc and visible light were both +0.30 V vs. Ag/AgCl. The flat band potential was roughly estimated by plotting the onset potential and the pH. The slope of the line was −60 mV, and the potential at pH = 0 was estimated to be 1.05 V (1.25 V) vs. Ag/AgCl (vs. SHE), which was close to the value for a pure CaFe2 O4 electrode reported previously [10]. In general, the onset potential is close to the flat-band potential and the valence band is close to the flat-band potential in p-type semiconductors. Fig. 7 shows the Mott–Schottky plot of the CFO (Fe/Ca = 1.9) electrode in 0.5 M H2 SO4 solution. The value of the flat band potential estimated from the plot was 1.19 V, which was in good accordance with the one determined from the onset potential (1.25 V). The valence band of CaFe2 O4 pallets was reported as around 1.3 V vs. SHE [10]. The onset potential of TiO2 (rutile) electrode in 0.1 M NaOH solution was −0.78 V. Fig. 8 shows the current-potential curve for a cell where p-type CFO (Fe/Ca = 1.9) and n-type TiO2 electrodes were connected in 0.1 M NaOH aqueous solution in the two chamber system under the Xe lamp illumination (full arc). The OCV was 1.09 V, and the shortcircuit current (V = 0) was 550 A. The difference between the two onset potentials of CaFe2 O4 (0.30 V) and TiO2 (−0.78 V) was 1.08 V, which was close to the OCV of the photocell in Fig. 8. The maximum power density was 0.108 mW cm−2 . Fig. 9a shows the amounts of hydrogen and oxygen generated in the photocell with CFO (Fe/Ca = 1.9) and TiO2 electrodes under short circuited condition, plotted as a function of the irradiation time. The amount of H2 evolved was approximately equivalent to a half of the electrons passing through the outer circuit, while the amount
Fig. 7. Mott–Schottky plot of the CFO (Fe/Ca = 1.9) electrode in 0.5 M H2 SO4 solution.
Fig. 8. Current-potential curves for a photocell with p-type CFO (Fe/Ca = 1.9) and n-type TiO2 electrodes in 0.1 M NaOH aqueous solution and powder density under a Xe lamp full arc illumination (electrode area of CFO: 2 cm2 , electrode area of TiO2 : 2 cm2 ).
of O2 evolved was equivalent to less than a quarter of the electron transport. The Faraday efficiencies for the H2 and O2 generation were 96–100% and 33–51%, which may be due to the oxidation of Ti substrate or oxygen absorption on the TiO2 /Ti electrode. The gas production rates decreased with the increasing irradiation time. Table 1 shows the ratio of hydrogen and oxygen evolved from the short circuited CFO–TiO2 electrodes. The ratio increased from 3.7 to 6.1 with the reaction time. The reason for the deviation of H2 /O2 ratio from 2.0 might be due to the photo-decomposition of CFO electrode. Fig. 9b shows the current-time curve for the CFO (Fe/Ca = 1.9)–TiO2 system. The photocurrent slowly decreased with Table 1 Ratio of hydrogen and oxygen evolved from the short circuited CFO–TiO2 electrodes. Reaction time [h]
H2 /O2 ratio
12 24 36 54
3.7 6.1 6.2 6.1
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Fig. 9. (a) Amounts of H2 and O2 gases generated from CFO (Fe/Ca = 1.9) and TiO2 electrodes system as a function of illumination time, and (b) current-time curve for the CFO-TiO2 system (electrode area of CFO: 2 cm2 , electrode area of TiO2 : 0.25 cm2 ).
detected under visible light illumination (>420 nm). If a n-type semiconductor with visible light response is used instead of the TiO2 electrode, hydrogen will be generated with this present system without any external voltage under visible light illumination. 4. Conclusions
Fig. 10. SEM image of CFO (Fe/Ca = 1.9) electrode after 52 h reaction. Table 2 Incident photon-to-current efficiency for the short-circuited CFO (Fe/Ca = 1.9)–TiO2 system under monochromatic light. Wavelength [nm]
Power density [mW/cm2 ]
IPCE [%]
320 350 370 400 420 450 470 500
3.917 4.207 4.757 8.725 9.894 10.769 11.185 9.557
2.02 1.20 0.66 0.06 0.02 0.01 0.01 0.01
the illumination time. Fig. 10 shows SEM image of the CFO electrode after a 52-h reaction. Many cracks were observed on the CFO surface, indicating that the CFO electrode slightly decomposed during the reaction, which may have led to the reduction of the photocurrent. Table 2 shows the incident photon-to-current efficiency for the short-circuited CFO (Fe/Ca = 1.9)–TiO2 system under monochromatic light. The efficiency at 320 nm was 2.02%, and decreased with increasing light wavelength. Hydrogen gas was not
Hydrogen and oxygen were generated from the p-type CFO (Fe/Ca = 1.9) and n-type TiO2 electrode system without any external voltage under illumination. It was found that the present CFO (Fe/Ca = 1.9) electrode showed superior performance to the pure CaFe2 O4 electrode. The ratio of H2 /O2 evolved from the CFO (Fe/Ca = 1.9)–TiO2 system after a 12-h reaction was 3.7. To the best of our knowledge, the present result is the closest to the complete water splitting using oxide p-type and n-type semiconductor electrodes. This present system using p-type and n-type semiconductor materials is a promising method of hydrogen production using solar energy. It is believed that this present system can form the basis of the future artificial photosynthesis system where hydrogen and oxygen are produced directly from water and sunlight. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Chemical Reviews 110 (2011) 6446. [3] R. Abe, M. Higashi, K. Domen, Journal of the American Chemical Society 132 (2010) 11828. [4] I. Cesar, A. Kay, J.A.G. Martinez, M. Grätzel, Journal of the American Chemical Society 128 (2006) 4582. [5] D. Yokoyama, T. Minegishi, K. Maeda, M. Katayama, J. Kubota, A. Yamada, M. Konagai, K. Domen, Electrochemistry Communications 12 (2010) 851. [6] G. Ma, T. Minegishi, D. Yokoyama, J. Kubota, K. Domen, Chemical Physics Letters 501 (2011) 619. [7] J.E. Turner, M. Hendewerk, G.A. Somorjai, Chemical Physics Letters 105 (1984) 581. [8] T. Inoue, T. Yamase, Chemistry Letters (1985) 869. [9] S. Ida, Y. Yamada, T. Matsunaga, H. Hagiwara, Y. Matsumoto, T. Ishihara, Journal of the American Chemical Society 132 (2010) 17343. [10] Y. Matsumoto, M. Omae, K. Sugiyama, E. Sato, Journal of Physical Chemistry 91 (1987) 577. [11] B.F. Decker, J.S. Kasper, Acta Crystallographica 10 (1957) 332.